A silver halide emulsion to be incorporated in a silver halide photographic material is normally subjected to chemical sensitization with various chemical substances to obtain a desired sensitivity, gradation, etc. Typical examples of chemical sensitization include sulfur sensitization, selenium sensitization, noble metal sensitization with gold or the like, reduction sensitization, and combinations thereof.
In recent years, high sensitivity, excellent graininess and high sharpness have been desired properties for silver halide photographic material. Furthermore, processing silver halide photographic materials at a higher speed than ever has also been desired. Thus, many improvements have been made in the above-mentioned sensitization processes.
In addition to the above-mentioned chemical sensitization processes, a new process has been proposed which comprises adding a so-called silver halide solvent as described later to the system upon chemical ripening to further improve the sensitivity.
However, this process is disadvantageous in that it causes fogging or a sensitivity change (mostly desensitization) during the storage of the light-sensitive material. Thus, this process makes it difficult to make the best use of the sensitizing effect and produces only an insufficient result.
In general, silver halide grains are formed by reacting an aqueous solution of a silver salt with an aqueous solution of a halide in an aqueous colloidal solution in a reaction vessel. The single jet process which comprises adding an aqueous solution of a silver salt to a mixture of a protective colloid such as gelatin and an aqueous solution of a halide in a reaction vessel with stirring over a certain period of time and the double jet process which comprises adding an aqueous solution of a halide and an aqueous solution of a silver salt to an aqueous solution of gelatin in a reaction vessel for certain periods of time, respectively. By comparison, the double jet process provides silver halide grains having a narrower grain size distribution than the single jet process. In the double jet process, the halide composition can be freely altered as the growth of the grains progresses.
It is known that the growth rate of silver halide grains largely depends on the concentration of silver ions (halogen ions) in the reaction solution, the concentration of the silver halide solvent, the distance between the grains, the grain size, etc. In particular, the lack of uniformity in the concentration of silver ions or halogen ions produced by the addition of an aqueous solution of a silver salt and an aqueous solution of a halide results in different growth rates, giving a non-uniformity in the resulting silver halide emulsion. In order to eliminate this problem, it is necessary to rapidly and uniformly mix and react the aqueous solution of the silver salt with the aqueous solution of the halide in the aqueous solution of the colloid so as to provide uniformity in the concentration of the silver ions or the halogen ions in the reaction vessel. In the conventional process which comprises adding an aqueous solution of a halide and an aqueous solution of a silver salt to the surface of an aqueous solution of a colloid in a reaction vessel, portions of higher halogen ion and silver ion concentrations are produced in the vicinity of the location at which each reaction solution is added. This results in difficulty in the preparation of uniform silver halide grains. Methods for eliminating such an uneven concentration distribution are disclosed in U.S. Pat. Nos. 3,415,650, and 3,692,283,and British Patent 1,323,464. In these methods, a reaction vessel is filled with an aqueous solution of a colloid. The reaction vessel is equipped with a rotary convex cylindrical hollow mixer having slits in the wall thereof (filled with an aqueous solution of a colloid, preferably composed of an upper chamber and a lower chamber partitioned by a disc in the vessel). The axis of rotation of the mixer is vertical. The aqueous solution of the halide and the aqueous solution of the silver salt are supplied into the mixer, which is rotating at a high speed, at the top and bottom open ends through feed pipes so that they are rapidly mixed and reacted with each other. (If there are two chambers in the mixer, the two aqueous solutions supplied into the respective chamber are first diluted with an aqueous solution of the colloid present therein, and then they are rapidly mixed and reacted with each other in the vicinity of the outlet slits.) The silver halide grains thus formed are then introduced into the aqueous solution of the colloid in the reaction vessel by the centrifugal force produced by the rotation of the mixer.
On the other hand, JP-B-55-l0545 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a method for eliminating an uneven concentration distribution to prevent non-uniform growth of grains. In this method, an aqueous solution of a halide and an aqueous solution of a silver salt are separately supplied into a mixer filled with an aqueous solution of the colloid in a reaction vessel filled with an aqueous solution of the colloid from the bottom open end of the mixer through feed pipes. These reaction solutions are rapidly agitated and mixed with each other by a lower agitator (turbine impeller) provided in the mixer to effect the growth of silver halide. The resulting silver halide grains are immediately introduced into the aqueous solution of the colloid in the reaction vessel from the upper open end of the mixer by an upper agitator provided above the lower agitator.
JP-A-57-92523 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a preparation method which is intended to eliminate such a non-uniformity in concentration. In this method, an aqueous solution of a halide and an aqueous solution of a silver salt are separately supplied into a mixer filled with an aqueous solution of a colloid in a reaction vessel filled with an aqueous solution of the colloid from a lower open end of the mixer. The two reaction solutions are diluted with the aqueous solution of the colloid and then rapidly mixed with each other by a lower agitator provided in the mixer. The resulting silver halide grains are immediately introduced into the aqueous solution of the colloid in the reaction vessel from an upper open end of the mixer. In this method and apparatus therefor, the two reaction solutions which have been, diluted with the aqueous solution of the colloid are passed through the clearance between the inner wall of the mixer and the tip of the agitator without being passed through the gaps between the impellers so that they are rapidly mixed and reacted with each other under a shearing force in the clearance to form silver halide grains.
These methods and apparatus can thoroughly eliminate the uneven distribution of concentration of silver ions and halogen ion in the reaction vessel. However, an uneven concentration distribution still exists in the mixer. In particular, a relatively large uneven concentration distribution exists in the vicinity of the nozzle through which the aqueous solution of the silver salt and the aqueous solution of the halide are supplied of the portion under the agitator and of the portions agitated. Furthermore, the silver halide grains supplied into the mixer together with the protective colloid are passed through these portions having an uneven concentration distribution. It should be particularly noted that the silver halide grains rapidly grow in these portions. In other words, these preparation methods and apparatus therefor are disadvantageous in that an uneven concentration distribution exists in the mixer, and the growth of grains takes place rapidly in the mixer, failing to accomplish the object of allowing uniform growth of the silver halide under conditions free of a concentration distribution difference.
In order to accomplish a more efficient mixing so as to eliminate the uneven concentration distribution of silver ions and halogen ions, additional attempts have been made. For example, a reaction vessel and a mixer are independently provided. An aqueous solution of a silver salt and an aqueous solution of a halide are supplied into the mixer where they are rapidly mixed with each other to effect the growth of silver halide grains. In a preparation method and apparatus disclosed in JP-A-53-37414 and JP-B-48-21045, an aqueous solution of a protective colloid (containing silver halide grains) is pumped from the bottom of a reaction vessel and circulated therein. A mixer is provided in the course of the circulation system. An aqueous solution of a silver salt and an aqueous solution of a halogen are supplied into the mixer where they are rapidly mixed with each other to effect the growth of silver halide grains. In a method disclosed in U.S. Pat. No. 3,897,935, an aqueous solution of a protective colloid (containing silver halide grains) is pumped from the bottom of a reaction vessel and circulated therein. An aqueous solution of a halide and an aqueous solution of a silver salt are pumped into the course of the circulation system. In a preparation method and apparatus disclosed in JP-A-53-47397, an aqueous solution of a protective colloid (containing silver halide grains) is pumped from the bottom of a reaction vessel and circulated therein. An aqueous solution of an alkali metal halide is first introduced into the circulation system. The aqueous solution of an alkali metal halide is diffused into the system until the system becomes uniform. Thereafter, an aqueous solution of a silver salt is introduced into and mixed with the system to form silver halide grains. These methods enable independent altering of the rate at which the aqueous solutions flow from the reaction vessel to the circulation system and the agitation efficiency of the mixer, making it possible to effect growth of grains under a condition of a more uniform concentration distribution. However, these methods are still disadvantageous in that the crystalline silver halide which has been delivered from the reaction vessel together with the protective colloid is subject to rapid growth at the inlet portion from which the aqueous solution of the silver salt and the aqueous solution of the halide are introduced into the system. Therefore, in these methods, it is impossible, in principle, to eliminate such a concentration distribution difference in the mixing portion or in the vicinity of the inlet portion. That is, the object of allowing uniform growth of silver halide under a condition free of concentration distribution cannot be accomplished.
In order to overcome these problems, the inventors disclosed silver halide grains having a completely uniform halogen distribution therein, no halide composition distribution between grains and/or no reduced silver produced upon the formation of grains or no distribution of reduced silver between grains, and a light-sensitive material comprising such silver halide grains in JP-A-1-183417, JP-A-1-183644 and JP-A-1-183645.